Many of the customs we practice to celebrate Halloween today have deep seeded roots in the traditions of ancient times.
Trick-or-treating for example, most probably originated from All Soul's Day parades that were held in England many years ago. Reason being is that during these parades, destitute peoples would plead for something to eat as they could not afford to buy this sustenance Other families in turn would present them with special pastries deemed "soul cakes" as a promise to pray for those in the family that had passed on to the afterlife.
Not to be forgotten, many families would also leave food and drink outside during All Hallows Eve and All Saint's Day to keep wandering spirits at bay. Some would say that this tradition could be where children dressing up as ghouls and goblins represent the wandering spirits and the candy/food given would coincide with that custom.
Which brings us to why we dress up in costumes today for both parties and the act of trick-or-treating. Long ago, during Celtic times, special ceremonies were held to signify the ending of Summer and the beginning of winter. This was called Samhain.
During these ancient festivities, Celts would wear costumes made from animal skins. The Celts believed that on this night, October 31st, the dead could cross over from their world back in to ours. To keep the spirits from recognizing them, they would dress in costumes to hide their true identities from the wandering spirits.
They believed that by wearing masks, that this would conceal them and make the roaming spirits think they were other kindred spirits simply wandering about and leave them at peace.
By incorporating the disguises and leaving food offerings outside their homes, people of the past believed it would keep restless wandering spirits distracted protecting themselves and their homes from becoming infested with unwanted spirt guests.
Obviously today, there are many other types of costumes we don on Halloween. Not just the traditional witches, goblins, and ghosts of the past.
All of this explains why in current times we wear costumes, throw parties, and go out trick-or-treating on Halloween. To date, Americans spend an estimated $6.9 billion every year celebrating Halloween. That's a whole lot of candy and costumes!